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The thermo-elastic properties and gel formation of polyvinyl alcohol in various media
L . N . VERKHOTINA et al.
1516
REFERENCES 1. Ye. V. KOCHETOV, A. A. BERLIN and N. S. YENIKOLOPYAN, Vysokomol. soyed. 8: 1022, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 6, 1122, 1966); Ye. V. KOCB[ETOV, M. A. MARKEVICH and N. S. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 180:143,1968 2. F. RANOGAETS, Ye. V. KOCHETOV, M. A. MARKEVICH and N. S. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR ~02: 642, 1972 3. M. A. MARKEVICH, N. F. KERDINA, Ye. V. KOCHETOV and N. 5. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 185: 125, 1969; N. MATHES and V. JAACKS, Makromol. Chemic 135: 49, 1970 4. IV[. A. MARKEVICH, L. K. PAKHOMOVA and N. S. YENIKOLOPYAN, Dokl. Akad. N a u k SSSR 187: 609, 1969; V. JAACKS and N. MATHES, Makromol. Chemic 131: 295, 1970 5. M. A. MARKEVICH, Ye. V. KOCHETOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. A13: 1033, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 5, 1163, 1971) 6. F. RANOGAETS, M. A. MARKEVICH, Ye. V. KOCHETOV and N. 5. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 200: 634, 1972 7. Ye. V. KOCHETOV, A. A. BERLIN, Ye. M. MASAL'SKAYA and N. S. YENIKOLOPYAN, Vysokomol. soyed. A12: 1118, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 5, 1264, 1970) 8. M. A. AKHM.AMET'EV, S. M. KAZAKOV, K. M. SOBOLEVSKII and V. N. SUMITEL'NOV, Avtometriya, No. 5, 34, 1971 9. Ye. V. KOCHETOV, A. A. BERLIN and N. 5. YENIKOLOPYAN, Vysokomol. soyed. 8: 1018, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 6, 1117, 1966) 10. D. T. MOWRY and R. R. MORNER, J. Am. Chem. See. 69: 1831, 1947 11. H. HIBBERT, Berichte 39: 160, 1906 12. C. G. OVERBERGER, Ye. M. PEARCE and N. MAYES, J. Polymer Sei. 34: 109, 1959
THE
THERM0-ELASTIC PROPERTIES AND GEL FORMATION OF POLYVINYL ALCOHOL IN VARIOUS MEDIA* L. N. VERKHOTINA, L. S. GEMBITSKII and YE. :N. GUBENKOVA Chemistry Research Institute, N. G. Chernyshevskii State University, Saratov
(Received 23 August 1971)
EARLIER studies [1, 2] described the thermodynamic rigidity of macromolecules to be a condition of the spontaneous rearrangement of polymers in bulk or when present as concentrated solutions. Flory [2] explained that polymers can crystallize during cooling because of the impossibility of macromolecules with limited flexibility taking on a chaotic distribution in the limited volume of sample. By applying the Flory model to amorphous polymers Gibbs and Di Martzio [3] concluded that such a polymer should also be subject to a second-order transi* Vysokomol. soyed. AI5: No. 6, 1350-1355, 1973.
Properties and gel formation of PVA in various media
1517
t i o n a t s o m e t e m p e r a t u r e , i.e. t o t h e g l a s s - l i k e s t a t e . V o l ' k e n s h t e i n [4] t r e a t e d t h i s t r a n s i t i o n as o n e f r o m a s y s t e m of n o n - c r y s t a l l i z i n g , c h a o t i c a l l y a r r a n g e d molecules to a part-oriented state. We observed a s u d d e n v o l u m e change w h e n cooling the
c o n c e n t r a t e s of
p o l y v i n y l a l c o h o l ( P V A ) i n a q u e o u s glycerol, a q u e o u s glycol or a q u e o u s d i e t h y l e n e glycol. Gel f o r m a t i o n t o o k p l a c e i n s u c h s y s t e m s [5]. W e w e r e t h e r e f o r e i n t e r e s t e d t o c o m p a r e t h e t h e r m o d y n a m i c r i g i d i t y of t h e :PVA c h a i n s i n v a r i o u s s o l v e n t s w i t h the gel-forming capacities of the p o l y m e r in these media.
EXPERIMENTAL PVA of Russian manufacture was used in this study, not fraetionated and having / 1 : 1 ~ 4 × 10 ~, and also fraetionated. M , = 7 5 × 10a (with a 1.4~o residual content of acetate groups). The swelling of the polymer samples was carried out in 50~/o aqueous solutions of glye(~rol, 18~/o aqueous glycol, 10~/o ethanol, 50~/o acetamide, or 50~oo ethanolamine (v/v). The conformational transitions of the PVA macromolecules in the various solvents were studied on the thermo-elasticity of films uniformly swollen in an excess of the reagent. The films were pI~)duced for this purpose from 4 % aqueous solutions at room temperature on a glass surface; their thickness was 50-100/2. The films--0.5 cm wide strips after swelling in wa~er for several days at room temperature--were immersed for 20 rain in a saturated terephthalic aldehyde solution, as described by Bashaw and Smith [6]. This produced a slightly crosslinked structure in the PVA (sample PVA-T). I n addition to this, the air-dried fihns were heated in a nitrogen atmosphere to 190°C/1 hr (sample ]?VA-K). This tempering gave rise to additional peaks on the X-ray (liffractogran~u [5]. We also examined the gelled films produced by cooling a 20~/o IPVA solution in an aqueous glycerol solution (1 : 1 by volume) (Sample PVA-S). The samples were fixed between two clamps, the lower end of the film could move to a t t a i n a constant sample length. Cathetometer type KM-6, having a 0.0005 cm measuring aecm'acy, was used to determine the sample dimensions. The upper clamp was connected to the lever of an analytical balance to measure to shrinkage moment for a given sample length. All the attachments were placed in a rectangular glass cell (vessel) containing the liquid used to produce the swelling. The temperature was kept constant by the circulation of temperature-controlled water. The tests were carried out in the temperature range 20-45°C, the relaxation of the samt)h~ to constant length and shrinkage it each testing temperature. On removing bhe load from the system it was set aside overnight and kept 2 hr at 45°C before the start of each test. Controls a t higher and lower temperatures showed the experimental points to fall ()n a straight line. To change the medium, the sample was placed for 1 day into water and the new medium was then added batchwise over 2 days. The film length was kept constant and its deformation was determined at a 35°C average temperature. The dependence of film (strip) length on stress and temperature was found to be a linear function in the studied range.
RESULTS T h e t h e r m o - e l a s t i c p r o p e r t i e s of t h e 1)VA n e t w o r k o b t a i n e d w i t h t e r e p h t h a l i c a l d e h y d e as c r o s s l i n k i n g a g e n t d u r i n g e q u i l i b r i u m s w e l l i n g i n w a t e r a n d t h e a q u e o u s s o l v e n t s o l u t i o n s w e r e a l r e a d y d e s c r i S e d b y o t h e r a u t h o r s [6]. T h e y
1518
L.N.
~rERKHOTINA e~ at.
also gave an analysis of the thermo-elastic behaviour of the polymeric network during a uniform swelling in an excess of solvent [7]. The ratio of the energy component f6 to the full force is described by
~=I--f\OT/p,L
6 (a'/,--l)'
(11
in which T--absolute temperature, (df/dT)e,L.--the particular temperature derivative of the force determined at the swelling equilibrium and at constant pressure P, and sample length L; fie--thermal expansion coefficient of the swelling film; a--relative deformation. Swelling in a solvent mixture having fie--0 will give an eqn. (1) identical in shape with that of Hoeve and Flory [8],
T/Bfh
fe
in which the term T (~f/c?T)e,L. is the entropic component f of the full force. On the basis of the thermodynamic determination of the energy force moment and of the statistical theory of highly elastic deformation, one can also determine the temperature coefficient of the unperturbed dimensions of the chain (d In h~/dT)
if], d In h 2
~=T
dT
(3)
in which h~--average squared distance between the chain ends of the network. Parameter (d In h~/dT) characterizes the chain flexibility of the network between its contact points. The rotation isomerism theory (1) makes it clear that the energetic force moment of the deformation stress, reduced for the constant volume of the sample, must be of a purely intramoleeular type. Confirmation for this is the independence of fe from the factors affecting the intermolecular reactions in the experiments by Cifferi, Hoeve and Flory [9, 10] with polyethylene, polydimethylsiloxane and poly-isobutylene. The thermomechanical properties of these polymers were therefore interpreted on the basis of the rotational isomerism theory which establishes the connection between fdf and the energy difference A U between gauche- and trans-isomers of the monomer units in the chain: fe_=
f
2 AU
(4)
2+gRT'
in which g--statistical weight of the rotation isomer, R--gas constant. In some specific cases one can write,
re~ f - RT
(4a)
Properties a n d gel f o r m a t i o n of P V A i n various m e d i a
1519
The thermal elongation coefficient flz, and thus also fls=3flL can be determined from the length of the swelling film as a function of temperature at several small and constant loads. The extrapolation to a zero load will give the temperature dependence of the undeformed swollen film length L 0. L O~ 177177
ZT.Z -
~
ZJ.7
3
I
JQ3
I
l
JzJ T,
FIG. 1. The l o n g i t u d i n a l changes of a swelling P V A - T (fraction) film as a f u n c t i o n of t e m p e r a t u r e in: / - - w a t e r ; 2 - - a q u e o u s glycerol (1 : 1); 3 - - a c e t a m i d e (1 : 1); 4 - - e t h a n o l a m i n e (1 : 1); 5 - - a PVA-S film in aqueous glycerol ( 1 : 1 ) .
The temperature dependence of Lo is shown for sample PVA-T in the studied media. The dependence of the contraction force on temperature is linear and depends on the properties of the medium used to swell the sample. The temper&ture dependence of f for PVA-T in water and in aqueous glycerol is reproduced in Fig. 2. It is linear and (~f/~T)p,L is independent ofa. The fe/f values calculated from eqn. (1) on the basis of the experimental fls-a and (#f/aT)p,~-values for a uniformly swollen PVA sample are given in the Table which also contains the A U and the d In h~/dT values calculated by using eqn. (4a). The results of the stress-deformation tests of the swelling networks were used to estimate the average molecular weight (mol. wt.) of the network Me, using the equation given by Bashaw [7] after modification for the Mc determination:
Mc=
pRT
,/
"),
(5)
1520
L . N . VERKHOTINA Ct al.
The value ~/~(h~)/(h~o)--ratio of the average quadratic distance between the chain ends in the network to that of the same chains in the free state, was taken to be unity, as done elsewhere [11], r--stress related to the transversal cross section of the non-swelling, undeformed network, p - - d r y polymer density equalling 1.28 g/cm a, vs--part of polymer by volume. The Mc of the gelled film was determined from the Treloar formula [12]: ~/c:
pRT
(a2--~t -1) ,
(6)
T
in which p--density of the gelled film equalling 1.25 g/cm a. f,g.
37 ~
16
C
24
6
7 8
J
303
l
l
3Z3
3g,Y
823
5
8 303
~ 323 7:,°K
~IO. 2. The temperature dependence of the shrinkage force f for: a - - P V A - T (fraction) in water; b - - i n aqueous glycerol; c--aqueous aeetaraide; at ~-values of: 1--1.10; 2--1.07; 3-- 1.05; 4-- 1'09; 5-- 1.08; 6-- 1.06; 7-- 1-03; 8-- 1-01.
A Heppler consistometer was used to study the strength of gels produced from 20% solutions of PVA fractions. We determined the minimum of stress causing the flow of the gels after maturing several days in the consistometer cell. I n contrast with the studies by Flory and co-workers [9, 10] we found that the value and the sign in front o f f e l l depended on the properties of the medium in which the gelling had taken place. As PVA possesses reactive functional groups which can form hydrogen bonds, the properties of the macromolecules must depend on whether such a bond exists between the monomer units and what type of bond it is, whether intra- or inter-molecular. The macromolecular conformations also determine the type of hydrogen bond present. The dependence of the sign in front of f e l l on the type of crosslinking (intermolecular in the case of crosslinking by thiodiacetaldehyde and intramolecular with formaldehyde) present in PVA fibres was investigated by Abe and Prins [13].
Properties and gel formation of PVA in various media
1521
S a k u r a d a a n d co-workers [14] studied t h e d e p e n d e n c e of t h e energetical force m o m e n t on t h e r e g u l a r i t y of s t r u c t u r e p r e s e n t in t)VA. As t h e r e is a different t y p e of h y d r o g e n b o n d p r e s e n t in iso- a n d s y n d i o t a c t i c :PVA, t h e d e p e n d e n c e of fell on t h e stereo-specificity indicates fe/f to be d e t e r m i n e d b y t h e n a t u r e of i n t e r a c t i o n s in t h e s y s t e m . T h e fe/f is a s s u m e d to be p o s i t i v e in a c o m p l e t e l y s y n d i o t a c t i c P V A ; it follows f r o m t h e T a b l e t h a t t h e fe/f values are p o s i t i v e in a h n o s t all t h e s t u d i e d media. This points to t h e f a c t t h a t t h e coiled c o n f o r m a tions of m o n o m e r units h a v e t h e m i n i m u m of free e n e r g y a n d this m a k e s one t h i n k t h a t a considerable n u m b e r of s y n d i o t a c t i c sequences exists in t h e studied PVA. THERMO-ELASTICITIES
OF
PVA S A M P L E S D U R I N G ( T : 308°K, ~ : 1-10)
EQUILIBRIAL
S~,VELLING
7 Polymer
PVA-T (fraction)
">I,
Solvent
Water 75
W'ater : Glycerol*
PVA-S PVA-K (not fractionated
75 40
PVA-T (not fractionated
40
Water : acetamide * Water : ethanolamine * Water : glycerol * Water : glycerol * Water : ethanol o/ (10/o) Water : glycol
i
(18%)
--,.19[ 048 ,.56[--290 1510 '
0.125 >30.00 25.00 1 .O0
0 . 5 8 [ 1 . 2 5 4.09]--770 l l l 0 0 [--0.70 --2.27 430 920 0 I 0.50 1.62 --310 1470 i
!
1.33i 4.30J-814
0.49 1 10, 0
0'77 0.05~
0
0.45
11 l0 t[/ 30.00 2.50 --470 1440t t -0-16! --30 1840ti No geli ling 1.46 -- 275 2840ti -} i
1 : lv/v t Acc. to [6].
*
T h e fairly flexible chains p r e s e n t as a result o f h y d r o p h o b i c r e a c t i o n in w a t e r (luring t h e swelling ( n e g a t i v e fiB) will coil a n d t h u s p r e v e n t t h e f o r m a t i o n of i n t e r m o l e c u l a r c o n t a c t points, so t h a t gel f o r m a t i o n will be hindered. T h e fe/f v a l u e is n e a r zero in a q u e o u s e t h a n o l a n d t h e chain dimensions r e m a i n a l m o s t t h e s a m e d u r i n g t e m p e r a t u r e variations. T h e A U is small a n d this indicates t h e lack o f correlation in t h e position of a d j a c e n t m o n o m e r units. Gelling does n o t t a k e place in this m e d i u m . T h e s t r o n g e s t gels were o b t a i n e d in s y s t e m P V A - a q u e o u s glycerol, in which t h e r e is a n o t i c e a b l y larger f r e q u e n c y of crosslinking, t h e fiB-values are m o r e p o s i t i v e a n d t h e d In h2/dT values larger. All this is e v i d e n c e of a considerable chain r i g i d i t y in t h e n e t w o r k as a result o f t h e p o l y m e r i n t e r a c t i o n w i t h t h e s o l v e n t a n d t h e f o r m a t i o n o f a d d i t i o n a l i n t e r m o l e c u l a r b o n d s w h e r e t h e r e are s y n d i o t a c t i c sequences. O t h e r g e l - f o r m a t i o n studies h a d s h o w n [11] t h a t t h e
1522
L. lh'. VERKKOTI~Aet al.
syndio~aetic sequences present in PVA are toqpositions where intormoleeulax bonds form. The positive fell values of PVA were also noted by others [13, 15]. The results obtained in the thermo-elasticity studies of films gelled ir~ solvents were similar to those obtained with PVA-T in aqueous glycerol and also with the heated film (PVA-K) swelling in the same medium. The thermo-elastic properties of the PV/k gel are thus determined by the macromolecular conformations in the solvent and by the intermolecular H-bonds. Furthermore, slightly crosslinked polymer networks can be used as models in studies of conformational changes and to estimate the thermodynamic rigidity of the network of chains present in the gel. The flexibility limits and an increase of the positive value of fell correlate here with the gel strength in aqueous solutions of hydroxyl-containing substances. An exception is the aqueous acetamide, for which fe/f~O, but the gels axe very strong in this medium. The fell dependence on deformation, when the latter becomes larger, is accompanied by a change of the sign in front of (~f]~T)r. (Fig. 2c). As fl,~--0, the sign change in front of re/f is due to (af/~T)L changes, the latter then equalling --(aS/tOL)T, i.e. the entropy drops during deformation. An analysis of the conformation changes in poly-electrolytes showed [1] those of the macromolecules to depend on the reactions of any component with the solvent which is capable of a specific reaction with the polymer. The entropy of an individual molecule can decrease as the chain rigidity increases during the formation of an intramolecular hydrogen bond as well as during that of a bond between the polymer and the solvent. Similar additional contacts between the last two can probably also exist in the system PVA-aqueous acetamide. The chain flexibility restrictions will result in strong gels forming. A similar dependence of fell on a exists also in the aqueous ethanolamine solution, but the number of contacts determined on the basis of Mc is much smaller and the gels are not strong. The gel formation by PVA in various media therefore depends on the conformational transitions during the polymer-solvent reactions. The limitations of chain flexibility result in the formation of a considerable number of intermolecular contacts and a network formation in the gel. CONCLUSIONS
(1) The energy force moment of swelling polyvinyl alcohol (PVA) samples was found to depend on solvent quality and on deformation. (2) Strong gels will form when chain flexibility is restricted and a certain number of intermolecular hydrogen bonds are produced. (3) The uniformly swelling PVA network, produced by crosslinking with terephthalic aldehyde, can be used as a model in the study of chain flexibility in the PVA gel produced by the same solvent. Trans/ated by K. A. ALLE~t
Features of stress relaxation in PAN fibres
1523
REFERENCES 1. T. M. BIRSHTEIN and O. B. PTITSYN, Konformatsii makromolekul (Macromolecular Conformations). Izd. "Nauka", 1964 2. P. FLORY, Prec. Royal Soc. A234: 60, 1956 3. J. GIBBS and E. DI MARZIO, J. Chem. Phys. 28: 373, 807, 1959 4. M. V. VOL'KENSHTEIN, Konfiguratsionnaya statistika polimernykh tsepei (Configurationals Statistics for Polymer Chains). Izd. Akad. Nauk SSSR, 1959 5. L. S. GEM~ITSKII, Ye. N. GUBENKOVA, L. N. VERKHOTINA and V. V. SPERANSKII, Vysokomol. soyed. A12: 259, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 2, 295, 1970) 6. J. BASHAW and K. J. SMITH Jr., J. Polymer Sci. 6, A-2: 1051, 1968 7. J. BASHAW and K. J. SMITH Jr., J. Polymer Sei. 6, A-2: 1041, 1968 8. C. A. HOEVE and P. J. FLORY, J. Polymer Sci. 60: 155, 1962 9. A. CIFERRI, C. HOEVE and P. FLORY, J. Am. Chem. Soc. 83: 1015, 1961 10. A. CIFERRI, Trans. F a r a d a y Soc. 57: 846, 1961 11. K. SHIBATANI, Polymer J. (Japan) 1: 348, 1970 12. L. TRELOAR, Fizika uprugosti kauehuka (Th~ Physics of Rubber Elasticity). Izd. inostr, lit., 1953 13. H. ABE and W. PRINS, J. Polymer Sci. CI: 527, 1963 14. J. SAKURADA, A. NAKAJIMA and K. SH1BATANI, Makromol. Chemie 87: 103, 1965 15. A. NAKAJIMA and N. YANAGAVA, J. Phys. Chem. 67: 654, 1963
FEATURES OF STRESS RELAXATION IN POLYACRYLONITRILE (PAN) FIBRES* V. P. KOSTROMIN a n d E. A. PAKSHVER All-Union Synthetic Fibres Research Institute
(Received 23 August 1971) A STRUCTURAL m o d e l w a s s u g g e s t e d [1], b e a r i n g i n m i n d t h e m i c r o - a n d m a c r o p o r o s i t y o f P A I ~ fibres, w h i c h a i m e d a t e x p l a i n i n g t h e i r b e h a v i o u r d u r i n g isom e t r i c h e a t i n g . T h e s t r e s s r e l a x a t i o n c h a r a c t e r i s t i c s a r e e x a m i n e d in t h i s s t u d y on P A I ~ fibres in t h e r a n g e 40-160°C, r e c o r d i n g t h e r e s p e c t i v e c u r v e s a n d calculating the constants of'the Kohlrausch equation from them. EXPERIMENTAL PAN fibres were used which were produced by the aqueous dimethylformamido method from an acrylonitrile copolymer of the following composition ( ~ ) : 92.5 acrylonitrile, 6 methyl mctlmerylato, 1-5 vinyl sulphonate. Fibres having different drawing histories wore studied. * Vysokomol. soyed. AI5: No. 6, 1356-1359, 1973.
1516
REFERENCES 1. Ye. V. KOCHETOV, A. A. BERLIN and N. S. YENIKOLOPYAN, Vysokomol. soyed. 8: 1022, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 6, 1122, 1966); Ye. V. KOCB[ETOV, M. A. MARKEVICH and N. S. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 180:143,1968 2. F. RANOGAETS, Ye. V. KOCHETOV, M. A. MARKEVICH and N. S. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR ~02: 642, 1972 3. M. A. MARKEVICH, N. F. KERDINA, Ye. V. KOCHETOV and N. 5. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 185: 125, 1969; N. MATHES and V. JAACKS, Makromol. Chemic 135: 49, 1970 4. IV[. A. MARKEVICH, L. K. PAKHOMOVA and N. S. YENIKOLOPYAN, Dokl. Akad. N a u k SSSR 187: 609, 1969; V. JAACKS and N. MATHES, Makromol. Chemic 131: 295, 1970 5. M. A. MARKEVICH, Ye. V. KOCHETOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. A13: 1033, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 5, 1163, 1971) 6. F. RANOGAETS, M. A. MARKEVICH, Ye. V. KOCHETOV and N. 5. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 200: 634, 1972 7. Ye. V. KOCHETOV, A. A. BERLIN, Ye. M. MASAL'SKAYA and N. S. YENIKOLOPYAN, Vysokomol. soyed. A12: 1118, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 5, 1264, 1970) 8. M. A. AKHM.AMET'EV, S. M. KAZAKOV, K. M. SOBOLEVSKII and V. N. SUMITEL'NOV, Avtometriya, No. 5, 34, 1971 9. Ye. V. KOCHETOV, A. A. BERLIN and N. 5. YENIKOLOPYAN, Vysokomol. soyed. 8: 1018, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 6, 1117, 1966) 10. D. T. MOWRY and R. R. MORNER, J. Am. Chem. See. 69: 1831, 1947 11. H. HIBBERT, Berichte 39: 160, 1906 12. C. G. OVERBERGER, Ye. M. PEARCE and N. MAYES, J. Polymer Sei. 34: 109, 1959
THE
THERM0-ELASTIC PROPERTIES AND GEL FORMATION OF POLYVINYL ALCOHOL IN VARIOUS MEDIA* L. N. VERKHOTINA, L. S. GEMBITSKII and YE. :N. GUBENKOVA Chemistry Research Institute, N. G. Chernyshevskii State University, Saratov
(Received 23 August 1971)
EARLIER studies [1, 2] described the thermodynamic rigidity of macromolecules to be a condition of the spontaneous rearrangement of polymers in bulk or when present as concentrated solutions. Flory [2] explained that polymers can crystallize during cooling because of the impossibility of macromolecules with limited flexibility taking on a chaotic distribution in the limited volume of sample. By applying the Flory model to amorphous polymers Gibbs and Di Martzio [3] concluded that such a polymer should also be subject to a second-order transi* Vysokomol. soyed. AI5: No. 6, 1350-1355, 1973.
Properties and gel formation of PVA in various media
1517
t i o n a t s o m e t e m p e r a t u r e , i.e. t o t h e g l a s s - l i k e s t a t e . V o l ' k e n s h t e i n [4] t r e a t e d t h i s t r a n s i t i o n as o n e f r o m a s y s t e m of n o n - c r y s t a l l i z i n g , c h a o t i c a l l y a r r a n g e d molecules to a part-oriented state. We observed a s u d d e n v o l u m e change w h e n cooling the
c o n c e n t r a t e s of
p o l y v i n y l a l c o h o l ( P V A ) i n a q u e o u s glycerol, a q u e o u s glycol or a q u e o u s d i e t h y l e n e glycol. Gel f o r m a t i o n t o o k p l a c e i n s u c h s y s t e m s [5]. W e w e r e t h e r e f o r e i n t e r e s t e d t o c o m p a r e t h e t h e r m o d y n a m i c r i g i d i t y of t h e :PVA c h a i n s i n v a r i o u s s o l v e n t s w i t h the gel-forming capacities of the p o l y m e r in these media.
EXPERIMENTAL PVA of Russian manufacture was used in this study, not fraetionated and having / 1 : 1 ~ 4 × 10 ~, and also fraetionated. M , = 7 5 × 10a (with a 1.4~o residual content of acetate groups). The swelling of the polymer samples was carried out in 50~/o aqueous solutions of glye(~rol, 18~/o aqueous glycol, 10~/o ethanol, 50~/o acetamide, or 50~oo ethanolamine (v/v). The conformational transitions of the PVA macromolecules in the various solvents were studied on the thermo-elasticity of films uniformly swollen in an excess of the reagent. The films were pI~)duced for this purpose from 4 % aqueous solutions at room temperature on a glass surface; their thickness was 50-100/2. The films--0.5 cm wide strips after swelling in wa~er for several days at room temperature--were immersed for 20 rain in a saturated terephthalic aldehyde solution, as described by Bashaw and Smith [6]. This produced a slightly crosslinked structure in the PVA (sample PVA-T). I n addition to this, the air-dried fihns were heated in a nitrogen atmosphere to 190°C/1 hr (sample ]?VA-K). This tempering gave rise to additional peaks on the X-ray (liffractogran~u [5]. We also examined the gelled films produced by cooling a 20~/o IPVA solution in an aqueous glycerol solution (1 : 1 by volume) (Sample PVA-S). The samples were fixed between two clamps, the lower end of the film could move to a t t a i n a constant sample length. Cathetometer type KM-6, having a 0.0005 cm measuring aecm'acy, was used to determine the sample dimensions. The upper clamp was connected to the lever of an analytical balance to measure to shrinkage moment for a given sample length. All the attachments were placed in a rectangular glass cell (vessel) containing the liquid used to produce the swelling. The temperature was kept constant by the circulation of temperature-controlled water. The tests were carried out in the temperature range 20-45°C, the relaxation of the samt)h~ to constant length and shrinkage it each testing temperature. On removing bhe load from the system it was set aside overnight and kept 2 hr at 45°C before the start of each test. Controls a t higher and lower temperatures showed the experimental points to fall ()n a straight line. To change the medium, the sample was placed for 1 day into water and the new medium was then added batchwise over 2 days. The film length was kept constant and its deformation was determined at a 35°C average temperature. The dependence of film (strip) length on stress and temperature was found to be a linear function in the studied range.
RESULTS T h e t h e r m o - e l a s t i c p r o p e r t i e s of t h e 1)VA n e t w o r k o b t a i n e d w i t h t e r e p h t h a l i c a l d e h y d e as c r o s s l i n k i n g a g e n t d u r i n g e q u i l i b r i u m s w e l l i n g i n w a t e r a n d t h e a q u e o u s s o l v e n t s o l u t i o n s w e r e a l r e a d y d e s c r i S e d b y o t h e r a u t h o r s [6]. T h e y
1518
L.N.
~rERKHOTINA e~ at.
also gave an analysis of the thermo-elastic behaviour of the polymeric network during a uniform swelling in an excess of solvent [7]. The ratio of the energy component f6 to the full force is described by
~=I--f\OT/p,L
6 (a'/,--l)'
(11
in which T--absolute temperature, (df/dT)e,L.--the particular temperature derivative of the force determined at the swelling equilibrium and at constant pressure P, and sample length L; fie--thermal expansion coefficient of the swelling film; a--relative deformation. Swelling in a solvent mixture having fie--0 will give an eqn. (1) identical in shape with that of Hoeve and Flory [8],
T/Bfh
fe
in which the term T (~f/c?T)e,L. is the entropic component f of the full force. On the basis of the thermodynamic determination of the energy force moment and of the statistical theory of highly elastic deformation, one can also determine the temperature coefficient of the unperturbed dimensions of the chain (d In h~/dT)
if], d In h 2
~=T
dT
(3)
in which h~--average squared distance between the chain ends of the network. Parameter (d In h~/dT) characterizes the chain flexibility of the network between its contact points. The rotation isomerism theory (1) makes it clear that the energetic force moment of the deformation stress, reduced for the constant volume of the sample, must be of a purely intramoleeular type. Confirmation for this is the independence of fe from the factors affecting the intermolecular reactions in the experiments by Cifferi, Hoeve and Flory [9, 10] with polyethylene, polydimethylsiloxane and poly-isobutylene. The thermomechanical properties of these polymers were therefore interpreted on the basis of the rotational isomerism theory which establishes the connection between fdf and the energy difference A U between gauche- and trans-isomers of the monomer units in the chain: fe_=
f
2 AU
(4)
2+gRT'
in which g--statistical weight of the rotation isomer, R--gas constant. In some specific cases one can write,
re~ f - RT
(4a)
Properties a n d gel f o r m a t i o n of P V A i n various m e d i a
1519
The thermal elongation coefficient flz, and thus also fls=3flL can be determined from the length of the swelling film as a function of temperature at several small and constant loads. The extrapolation to a zero load will give the temperature dependence of the undeformed swollen film length L 0. L O~ 177177
ZT.Z -
~
ZJ.7
3
I
JQ3
I
l
JzJ T,
FIG. 1. The l o n g i t u d i n a l changes of a swelling P V A - T (fraction) film as a f u n c t i o n of t e m p e r a t u r e in: / - - w a t e r ; 2 - - a q u e o u s glycerol (1 : 1); 3 - - a c e t a m i d e (1 : 1); 4 - - e t h a n o l a m i n e (1 : 1); 5 - - a PVA-S film in aqueous glycerol ( 1 : 1 ) .
The temperature dependence of Lo is shown for sample PVA-T in the studied media. The dependence of the contraction force on temperature is linear and depends on the properties of the medium used to swell the sample. The temper&ture dependence of f for PVA-T in water and in aqueous glycerol is reproduced in Fig. 2. It is linear and (~f/~T)p,L is independent ofa. The fe/f values calculated from eqn. (1) on the basis of the experimental fls-a and (#f/aT)p,~-values for a uniformly swollen PVA sample are given in the Table which also contains the A U and the d In h~/dT values calculated by using eqn. (4a). The results of the stress-deformation tests of the swelling networks were used to estimate the average molecular weight (mol. wt.) of the network Me, using the equation given by Bashaw [7] after modification for the Mc determination:
Mc=
pRT
,/
"),
(5)
1520
L . N . VERKHOTINA Ct al.
The value ~/~(h~)/(h~o)--ratio of the average quadratic distance between the chain ends in the network to that of the same chains in the free state, was taken to be unity, as done elsewhere [11], r--stress related to the transversal cross section of the non-swelling, undeformed network, p - - d r y polymer density equalling 1.28 g/cm a, vs--part of polymer by volume. The Mc of the gelled film was determined from the Treloar formula [12]: ~/c:
pRT
(a2--~t -1) ,
(6)
T
in which p--density of the gelled film equalling 1.25 g/cm a. f,g.
37 ~
16
C
24
6
7 8
J
303
l
l
3Z3
3g,Y
823
5
8 303
~ 323 7:,°K
~IO. 2. The temperature dependence of the shrinkage force f for: a - - P V A - T (fraction) in water; b - - i n aqueous glycerol; c--aqueous aeetaraide; at ~-values of: 1--1.10; 2--1.07; 3-- 1.05; 4-- 1'09; 5-- 1.08; 6-- 1.06; 7-- 1-03; 8-- 1-01.
A Heppler consistometer was used to study the strength of gels produced from 20% solutions of PVA fractions. We determined the minimum of stress causing the flow of the gels after maturing several days in the consistometer cell. I n contrast with the studies by Flory and co-workers [9, 10] we found that the value and the sign in front o f f e l l depended on the properties of the medium in which the gelling had taken place. As PVA possesses reactive functional groups which can form hydrogen bonds, the properties of the macromolecules must depend on whether such a bond exists between the monomer units and what type of bond it is, whether intra- or inter-molecular. The macromolecular conformations also determine the type of hydrogen bond present. The dependence of the sign in front of f e l l on the type of crosslinking (intermolecular in the case of crosslinking by thiodiacetaldehyde and intramolecular with formaldehyde) present in PVA fibres was investigated by Abe and Prins [13].
Properties and gel formation of PVA in various media
1521
S a k u r a d a a n d co-workers [14] studied t h e d e p e n d e n c e of t h e energetical force m o m e n t on t h e r e g u l a r i t y of s t r u c t u r e p r e s e n t in t)VA. As t h e r e is a different t y p e of h y d r o g e n b o n d p r e s e n t in iso- a n d s y n d i o t a c t i c :PVA, t h e d e p e n d e n c e of fell on t h e stereo-specificity indicates fe/f to be d e t e r m i n e d b y t h e n a t u r e of i n t e r a c t i o n s in t h e s y s t e m . T h e fe/f is a s s u m e d to be p o s i t i v e in a c o m p l e t e l y s y n d i o t a c t i c P V A ; it follows f r o m t h e T a b l e t h a t t h e fe/f values are p o s i t i v e in a h n o s t all t h e s t u d i e d media. This points to t h e f a c t t h a t t h e coiled c o n f o r m a tions of m o n o m e r units h a v e t h e m i n i m u m of free e n e r g y a n d this m a k e s one t h i n k t h a t a considerable n u m b e r of s y n d i o t a c t i c sequences exists in t h e studied PVA. THERMO-ELASTICITIES
OF
PVA S A M P L E S D U R I N G ( T : 308°K, ~ : 1-10)
EQUILIBRIAL
S~,VELLING
7 Polymer
PVA-T (fraction)
">I,
Solvent
Water 75
W'ater : Glycerol*
PVA-S PVA-K (not fractionated
75 40
PVA-T (not fractionated
40
Water : acetamide * Water : ethanolamine * Water : glycerol * Water : glycerol * Water : ethanol o/ (10/o) Water : glycol
i
(18%)
--,.19[ 048 ,.56[--290 1510 '
0.125 >30.00 25.00 1 .O0
0 . 5 8 [ 1 . 2 5 4.09]--770 l l l 0 0 [--0.70 --2.27 430 920 0 I 0.50 1.62 --310 1470 i
!
1.33i 4.30J-814
0.49 1 10, 0
0'77 0.05~
0
0.45
11 l0 t[/ 30.00 2.50 --470 1440t t -0-16! --30 1840ti No geli ling 1.46 -- 275 2840ti -} i
1 : lv/v t Acc. to [6].
*
T h e fairly flexible chains p r e s e n t as a result o f h y d r o p h o b i c r e a c t i o n in w a t e r (luring t h e swelling ( n e g a t i v e fiB) will coil a n d t h u s p r e v e n t t h e f o r m a t i o n of i n t e r m o l e c u l a r c o n t a c t points, so t h a t gel f o r m a t i o n will be hindered. T h e fe/f v a l u e is n e a r zero in a q u e o u s e t h a n o l a n d t h e chain dimensions r e m a i n a l m o s t t h e s a m e d u r i n g t e m p e r a t u r e variations. T h e A U is small a n d this indicates t h e lack o f correlation in t h e position of a d j a c e n t m o n o m e r units. Gelling does n o t t a k e place in this m e d i u m . T h e s t r o n g e s t gels were o b t a i n e d in s y s t e m P V A - a q u e o u s glycerol, in which t h e r e is a n o t i c e a b l y larger f r e q u e n c y of crosslinking, t h e fiB-values are m o r e p o s i t i v e a n d t h e d In h2/dT values larger. All this is e v i d e n c e of a considerable chain r i g i d i t y in t h e n e t w o r k as a result o f t h e p o l y m e r i n t e r a c t i o n w i t h t h e s o l v e n t a n d t h e f o r m a t i o n o f a d d i t i o n a l i n t e r m o l e c u l a r b o n d s w h e r e t h e r e are s y n d i o t a c t i c sequences. O t h e r g e l - f o r m a t i o n studies h a d s h o w n [11] t h a t t h e
1522
L. lh'. VERKKOTI~Aet al.
syndio~aetic sequences present in PVA are toqpositions where intormoleeulax bonds form. The positive fell values of PVA were also noted by others [13, 15]. The results obtained in the thermo-elasticity studies of films gelled ir~ solvents were similar to those obtained with PVA-T in aqueous glycerol and also with the heated film (PVA-K) swelling in the same medium. The thermo-elastic properties of the PV/k gel are thus determined by the macromolecular conformations in the solvent and by the intermolecular H-bonds. Furthermore, slightly crosslinked polymer networks can be used as models in studies of conformational changes and to estimate the thermodynamic rigidity of the network of chains present in the gel. The flexibility limits and an increase of the positive value of fell correlate here with the gel strength in aqueous solutions of hydroxyl-containing substances. An exception is the aqueous acetamide, for which fe/f~O, but the gels axe very strong in this medium. The fell dependence on deformation, when the latter becomes larger, is accompanied by a change of the sign in front of (~f]~T)r. (Fig. 2c). As fl,~--0, the sign change in front of re/f is due to (af/~T)L changes, the latter then equalling --(aS/tOL)T, i.e. the entropy drops during deformation. An analysis of the conformation changes in poly-electrolytes showed [1] those of the macromolecules to depend on the reactions of any component with the solvent which is capable of a specific reaction with the polymer. The entropy of an individual molecule can decrease as the chain rigidity increases during the formation of an intramolecular hydrogen bond as well as during that of a bond between the polymer and the solvent. Similar additional contacts between the last two can probably also exist in the system PVA-aqueous acetamide. The chain flexibility restrictions will result in strong gels forming. A similar dependence of fell on a exists also in the aqueous ethanolamine solution, but the number of contacts determined on the basis of Mc is much smaller and the gels are not strong. The gel formation by PVA in various media therefore depends on the conformational transitions during the polymer-solvent reactions. The limitations of chain flexibility result in the formation of a considerable number of intermolecular contacts and a network formation in the gel. CONCLUSIONS
(1) The energy force moment of swelling polyvinyl alcohol (PVA) samples was found to depend on solvent quality and on deformation. (2) Strong gels will form when chain flexibility is restricted and a certain number of intermolecular hydrogen bonds are produced. (3) The uniformly swelling PVA network, produced by crosslinking with terephthalic aldehyde, can be used as a model in the study of chain flexibility in the PVA gel produced by the same solvent. Trans/ated by K. A. ALLE~t
Features of stress relaxation in PAN fibres
1523
REFERENCES 1. T. M. BIRSHTEIN and O. B. PTITSYN, Konformatsii makromolekul (Macromolecular Conformations). Izd. "Nauka", 1964 2. P. FLORY, Prec. Royal Soc. A234: 60, 1956 3. J. GIBBS and E. DI MARZIO, J. Chem. Phys. 28: 373, 807, 1959 4. M. V. VOL'KENSHTEIN, Konfiguratsionnaya statistika polimernykh tsepei (Configurationals Statistics for Polymer Chains). Izd. Akad. Nauk SSSR, 1959 5. L. S. GEM~ITSKII, Ye. N. GUBENKOVA, L. N. VERKHOTINA and V. V. SPERANSKII, Vysokomol. soyed. A12: 259, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 2, 295, 1970) 6. J. BASHAW and K. J. SMITH Jr., J. Polymer Sci. 6, A-2: 1051, 1968 7. J. BASHAW and K. J. SMITH Jr., J. Polymer Sei. 6, A-2: 1041, 1968 8. C. A. HOEVE and P. J. FLORY, J. Polymer Sci. 60: 155, 1962 9. A. CIFERRI, C. HOEVE and P. FLORY, J. Am. Chem. Soc. 83: 1015, 1961 10. A. CIFERRI, Trans. F a r a d a y Soc. 57: 846, 1961 11. K. SHIBATANI, Polymer J. (Japan) 1: 348, 1970 12. L. TRELOAR, Fizika uprugosti kauehuka (Th~ Physics of Rubber Elasticity). Izd. inostr, lit., 1953 13. H. ABE and W. PRINS, J. Polymer Sci. CI: 527, 1963 14. J. SAKURADA, A. NAKAJIMA and K. SH1BATANI, Makromol. Chemie 87: 103, 1965 15. A. NAKAJIMA and N. YANAGAVA, J. Phys. Chem. 67: 654, 1963
FEATURES OF STRESS RELAXATION IN POLYACRYLONITRILE (PAN) FIBRES* V. P. KOSTROMIN a n d E. A. PAKSHVER All-Union Synthetic Fibres Research Institute
(Received 23 August 1971) A STRUCTURAL m o d e l w a s s u g g e s t e d [1], b e a r i n g i n m i n d t h e m i c r o - a n d m a c r o p o r o s i t y o f P A I ~ fibres, w h i c h a i m e d a t e x p l a i n i n g t h e i r b e h a v i o u r d u r i n g isom e t r i c h e a t i n g . T h e s t r e s s r e l a x a t i o n c h a r a c t e r i s t i c s a r e e x a m i n e d in t h i s s t u d y on P A I ~ fibres in t h e r a n g e 40-160°C, r e c o r d i n g t h e r e s p e c t i v e c u r v e s a n d calculating the constants of'the Kohlrausch equation from them. EXPERIMENTAL PAN fibres were used which were produced by the aqueous dimethylformamido method from an acrylonitrile copolymer of the following composition ( ~ ) : 92.5 acrylonitrile, 6 methyl mctlmerylato, 1-5 vinyl sulphonate. Fibres having different drawing histories wore studied. * Vysokomol. soyed. AI5: No. 6, 1356-1359, 1973.